1. Field of Invention
The present invention relates to a semiconductor device by gallium nitride compound. More particularly, the present invention relates to a structure capable of applying on a gallium nitride laser diode device for increasing a thickness of a surface epitaxial layer. In the specification, the surface epitaxial layer is referring to a patterned epitaxial layer in the gallium nitride laser diode device.
2. Description of Related Art
The typical optoelectronics device is a hetero structure. Since the crystal structure between the hetero epitaxial layers is not matched and the thermal expansion coefficients are also different, a strain energy exists at the interface. During the fabrication process or in operation of the device, this strain energy will be partially released by a form of dislocation or other defect forms. When the thickness of the epitaxial layer is greater than a certain critical value, the material then release the energy by a crack form, resulting in a crack of the epitaxial layer. For this consideration, the hetero epitaxial crystal layer cannot be grown in overlarge thickness. If the thickness of the epitaxial layer is large, a poor function of the device or even a severe damage would occur due to the crack. However, if the thickness of the epitaxial layer is not sufficient large, a poor performance may also occur.
In the conventional technology, it has several methods can reduce the problems caused by the strain energy in the hetero structure. For example, (1) a method of epitaxial lateral overgrowth (ELOG) is proposed. In addition, (2) a buffer layer is also introduced, wherein an aluminum nitride layer or a gallium nitride layer in a rather small thickness is formed on a substrate at low temperature, to serve as a buffer layer, so as to reduce the problem of not being match for the crystal lattice between the epitaxial layer and the substrate. As a result, the condition for subsequently growing the gallium nitride at high temperature is improved and the quality of epitaxial crystal structure is also improved. In addition, (3) a strain layer superlattice is also proposed.
The conventional gallium nitride device in the current status is mostly using the C-plane of the sapphire (Al2O3) as the substrate. It has about an amount of 16% in lattice mismatch existing between the substrate and the gallium nitride epitaxial crystal, resulting in a rather large strain energy inside the gallium nitride thin film, which is grown on the sapphire, in which the density of the dislocation is high up to 109 1011/cm2. The foregoing technology can only be used for solving the strain energy or effect, which are caused by epitaxial layer between substrate and the gallium nitride, or inside the device.
Taking the gallium nitride laser diode as an example, the application for the foregoing conventional technologies is basically limited to the epitaxial layer under the active layer. The epitaxial layer above the active layer still has the problem of hetero material structure. In other words, the conventional method still cannot effectively solve the problem of crack in the epitaxial layer above the active layer.
FIG. 1 is a cross-sectional view, schematically illustrating the structure of a conventional gallium nitride laser diode, which is sequentially formed with a substrate 101, a buffer layer 102 with gallium nitride compound semiconductor formed at relative low temperature, an N-type gallium nitride compound semiconductor layer 103, a set of bar mask 104, an N-type gallium nitride compound semiconductor layer 105, a heavily doped layer 106, an N-type gallium nitride compound semiconductor superlattice cladding layer 107, an N-type gallium nitride compound semiconductor light guiding layer 108, a gallium nitride compound semiconductor active layer 109, a P-type gallium nitride compound semiconductor cap layer 110, a P-type gallium nitride compound semiconductor light guiding layer 111, a P-type gallium nitride compound semiconductor superlattice cladding layer 112, and a P-type metal electrode contact layer 113. In order to improve the quality of the epitaxial crystal and prevent the crack from occurring, the conventional structure for the gallium nitride laser diode structure has included the low-temperature buffer layer 102, the method of ELOG (104), the strain layer super lattice structure (107, 112). However, these kinds of technologies for reducing the strain and the defects is not effective with respect to strain energy existing in the surface hetero epitaxial layer. FIG. 2, is a picture, schematically illustrating the crack occurring on the surface of the epitaxial layer for the conventional gallium nitride laser diode structure.
On the gallium nitride laser diode structure, the cladding layer is usually formed from Alx Ga1-x N, where if the quantity of X is higher, then the refraction index is smaller, the energy gap is larger, and the light confinement is better. As a result; the lattice mismatch is larger, thus, the thickness can not be overlarge. If the Alx Ga1-x N layer is too thick, the crack is then easily occurring, and causes a failure of device. However, if the cladding layer is not sufficiently thick, the effect of light confinement then is getting worse, and the performance of device is then poor. Thus, it is a difficult issue for fabrication that how to control the thickness of epitaxial layer and the composition, so as to reduce the cracking of the epitaxial layer and improve the performance of the device.